1. Technical Field
This invention relates generally to dot printers, and more particularly to internal means to protect the print head when power is applied in the absence of data and logic signals.
2. Background Art
Prior art print heads include a first electrical connection for both the LED and the logic power supplies, and a second connection for the data and logic signals. A central processing unit (CPU) senses when the power is turned on, and sends a reset signal to the print head over the second connection to place the LED's in a safe state. If the second connection is not made, or if the wire to the connection is broken when power is applied to the print head via the first connection, the logic comes up in an unknown, scrambled state and the CPU does not know that the second connection is not made. Any LED's that were "on" would be damaged by the application of power.
Another adverse situation occurs upon so-called cycle dropout. If there is a temporary power loss or drop to the print head, the logic may become scrambled so that some LED's may not be in a safe state. Normally, the CPU is programed to sense a power drop and to send a reset signal to the print head to bring the LED's to a known state so that there is no damage when full power is again applied. However, the logic on the print head may be more sensitive than the CPU so that a power drop sufficient to scramble the logic on the print head may be insufficient to cause the CPU to send a reset signal. Once again, those LED's that are not in their "safe" state will be damaged when full power is again applied.